1. Field of the Invention
The present invention relates to a normal conduction attraction type magnetic levitation apparatus which supports a levitation body without any contact.
2. Description of the Related Art
The normal conduction attraction type magnetic levitation apparatus, producing no noise or dust, has been already adopted for actual purposes in railroad such as high-speed surface transport (HSST), trans rapid and transport system in clean rooms of semiconductor factory.
Jpn. Pat. Appln. KOKAI Publication No. 2001-19286 has disclosed adoption of the magnetic levitation apparatus for the guide unit for a cage of the elevator. Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-303079 has disclosed adoption of the magnetic levitation apparatus for a door.
In this magnetic levitation apparatus, with electromagnets opposed to ferromagnetic members, the levitation body is levitated using attraction force generated with respect to the ferromagnetic members by excitation of the electromagnets. Thus, basically, the magnetic levitation system is instable and some measures for stabilizing it are necessary. Generally, a gap length of the levitation body is detected using a gap sensor and its result is fed back to the driving system for achieving the stabilization.
However, to detect the gap length accurately, it is necessary to obtain stable signals from the gap sensor. Unless the sensor target is controlled appropriately, noise at the time of gap length detection is superimposed on sensor signals thereby affecting the levitation control. Consequently, vibration is generated in a structure supporting the levitation body and ferromagnetic member.
The sensor target is a guide rail for supporting the cage in case of the elevator. A cage magnetic levitation apparatus is provided and the cage is levitated by this magnetic levitation apparatus and moved. If signals of the gap sensors are disturbed by joints of the guide rail, sometimes the cage can be shaken.
To stabilize the magnetic levitation system, the sensor target needs to be controlled appropriately, whereby a surplus cost being necessary. Further, a resonance preventive measure for the levitation body is needed, thereby the system being enlarged and complicated.
To solve these problems, various methods, which do not need any gap sensor, have been proposed.
For example, a method for estimating the gap length using an observer (state observing unit) from an exciting current of an electromagnet has been disclosed in “Research for Practical Application of a Deviation Sensorless Magnetic Bearing”, written by MIZUNO et al., in Bulletin D issued by Institute of Electrical Engineers of Japan, 116, No. 1, 35 (1996).
Further, a thesis “AC Magnetic Levitation using a Differential Feedback Type Power Amplifier”, written by MORIYAMA, in Report No. 1215 by Institute of Electrical Engineers of Japan has disclosed a method which includes gap information in a phase difference between an exciting voltage and an exciting current of an electromagnet generated by magnetic levitation so as to feed back this gap information to the exciting voltage.
Additionally, a thesis “Self Sensing Magnetic Levitation using Hysteresis Amplifier”, Bulletin issued by Society of Instrument and Control Engineers, 32, No. 7, 1043 (1996) has disclosed a method in which the exciting current value of an electromagnet is compared with a reference value by means of a hysteresis comparator and if the exciting current is larger than the reference value, the exciting voltage is switched to negative and if smaller, the exciting voltage is switched to positive so as to make the switching frequency proportional to the gap length.
However, the above-described solving means cannot be applied to a case where the observer is used. The reason is that the observer is introduced from the linear model of a magnetic levitation system in a levitation state and thus, it cannot estimate a gap length when it is not in the levitation state. Therefore, controls at the time of levitation start become difficult and further, if a levitation body comes into a contact with other structure, it cannot be restored to the levitation state again.
If the exciting voltage of the electromagnet is controlled by physical quantity including gap information, the levitation control system is of non-linear system. Thus, if variations in electric resistance are generated in the electromagnet coil due to a change in mass of the levitation body or a rise in temperature by excitation, the levitation state cannot be maintained.
To cope with such a problem, a method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-204609 is available. This concerns a sensorless method for estimating the gap length by means of the observer from the exciting current of the electromagnet, in which when the levitation body is not in a levitation state, the integrator of the observer is initialized by detecting a contact of the levitation body and the gap length at the time of a contact is estimated geometrically from the contact condition of the levitation body. Then, an initial value is given to the integrator of the observer based on this estimated value so as to restore the levitation body to the levitation state.
However, if this method is applied to the zero power control disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-102105, a following problem occurs.
That is, when the levitation body is a normal levitation state, the exciting current of the electromagnet is converged to zero and therefore, there is no problem. However, if a large external force is applied to the levitation body for a long period of time, transient control current continues to flow to a coil of the electromagnet, so that the temperature of the coil is raised. In association with this temperature rise, the electric resistance of the coil is increased, so that the output error of the observer, which estimates the gap length from the exciting current, is increased. As a result, it becomes gradually difficult to maintain the levitation state and thus the levitation body comes into a contact with a supporting member.
In the meantime, if the levitation body comes into a contact with the supporting member, restoration control for restoring to the levitation state is attained. However, even if the levitation body is restored to the levitation state, the levitation body comes into a contact again because the error of the estimated value of the gap length is large and the contact state and the levitation state are repeated alternately.
Because a large control current continues to flow to the electromagnet under this condition, the coil resistance value of the electromagnet is further increased, and finally, it comes that with the levitation body kept in contact with the supporting member, the exciting current continues to flow. If the exciting current continuing to flow is large, not only the reliability of the levitation state is deteriorated but also, the electromagnet can ignite.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 2005-117705 has proposed a method in which the sensorless magnetic levitation control is carried out by measuring the coil resistance value of the electromagnet and parameters of the observer are varied based on the measured resistance value.
Further, if the transient exciting current continues to flow to the electromagnet, not only the coil resistance value is increased but also the offset voltage is varied in association with a rise of the temperature. This variation of the offset voltage increases the output error of the observer which estimates the gap length as well as the variation of the coil resistance value.
To cope with this problem, an offset compensation amount is added to the exciting voltage which turns the velocity estimated value of the observer to zero so as to suppress the output error of the observer, as shown Jpn. Pat. Appln. KOKAI Publication No. 2006-325303.
Even if the above-described countermeasure is taken, no accurate resistance value can be measured if the offset voltage is mixed with the exciting voltage, because the resistance value of the coil used within the observer is calculated from the exciting voltage and a DC component of the exciting current.
To avoid this problem, Jpn. Pat. Appln. KOKAI Publication No. 2007-259521 has proposed a method in which two values, a zero value and a non-zero value are set as the coil current target value and if the target value is zero, the offset voltage is estimated to measure a coil resistance value accurately.
However, even if these countermeasures are taken, a slight delay is unavoidable in an estimated value of the levitation state in the sensorless magnetic levitation control when a sudden variation is generated in an actual levitation state. Thus, if the levitation state is varied at an unexpected velocity, the stability of the system cannot be compensated.
Particularly, if the sensorless magnetic levitation control is applied to transport system or traffic system in order to avoid the resonance of any structure, its low reliability is a problem.
As described above, in a conventional magnetic levitation apparatus, the gap sensor and sensor target are necessary in order to achieve a stable levitation state of the levitation body. However, if the gap sensors are used, the structure can be vibrated by noise components in the sensor signals and thus, a means for suppressing them is required. For the reason, the apparatus becomes enlarged and complicated, thereby increasing manufacturing cost.
If to avoid this problem, the gap length is estimated without use of any gap sensor so as to execute the feedback control (sensorless magnetic levitation control), the reliability of the levitation control is lowered as compared with a case where the gap sensor is used, due to a time delay for the reason of estimation of the gap length.